Radiation detector for wide-temperature-range operation



R. M'oLeTo'R 3,426,232

RADIATION DETECTOR FOR WIDE-TEMPERATURE-RANGE OPERATION Feb. 4, `1969 Filed March 19, 1965 0 M 3 6 d .J 4\l @mi 2 1| 8 32 v INVENTOR.

R0 5ML/wo /rp/ United States Patent O "ice 9 Claims ABSTRACT OF THE DISCLOSURE A flowtype proportional counter for use in gas chromatography employs a tetrafluoroethylene insulator for support of the anode wire. The insulator is bevelled and seated in a correspondingly bevelled end of the tubular cathode. A spring maintains the gas seal despite the elevated temperatures encountered in each period of use.

This invention relates to radiation detectors for operation under a wide range of temperature conditions. In particular, the invention relates to a construction whereby the integrity of the gas-seal of a gas-containing radiation detector may be maintained under conditions of temperature, and cycling of temperature, extending substantially beyond the limits heretofore obtainable with constructions of comparable simplicity.

Simple constructions for radiation detectors of the gasiilled type commonly employ one of the electrodes to form the body of the gas enclosure in which the ionization used for detection occurs, with the other electrode being mounted within the enclosure and connected to the exterior through an insulator extending out through the wall of the enclosure formed by the first electrode, the insulator -being constructed and installed in such a manner as to constitute a gas seal. A large variety of particular constructions of this type are well known and have long been in use. In general, the complexity of the structure of the insulator and associated mounting rings, clamps, etc., varies substantially with the pressure differential against which the seal is made. One common form of detector in wide-spread use is the flow counter, in which the gas is constantly replenished, being maintained at a pressure only very slightly greater than atmospheric pressure. Such counters, normally operated as proportional counters or Geiger counters, are of extremely simple construction, largely because of the relative simplicity of the insulator construction with which a seal can be made, in addition to the fact that in some cases (employing non-radioactive gas, as is normal) the matter of integrity of the seal may be immaterial, since the gas is constantly vented to the ambient atmosphere in any event.

Early uses of ow counters were primarily in the counting of beta-particles and alpha-particles from solid and liquid samples, the operation at substantially atmospheric pressure being required to make practical the introducing of the sample directly into the counter, or separated from the counting volume by a suliciently thin window A further use of flow counters, involving still simpler construction, is in the detection and measurement of radioactivity of the flow gas itself, in which case there is no necessity for provision of either a window or any gaslock or other means of introducing a sample into the counter. In such case, the entire detector structure may consist of a simple tubular cathode, an axial anode wire and suitable provisions for sealing the ends of the tubular cathode, mounting the anode, and attaching of a gas inlet and outlet. Counters of this general construction have long been used for a number of purposes, particularly in measurements on carbon-14 and tritium, both of which are 3,426,232 Patented Feb. 4, 1969 easily employed for the tagging of various organic compounds.

One application of such flow counters is in connection with gas chromatography. There are of course a number of types of detectors used for indication of composition of the exit gas from a chromatograph column, depending upon the particular requirements of the measurement, or type of measurement, involved. Various chemical or physical properties of the exit gas may be monitored in producing the spectrum indication. One important type of such detection employs the ow-type radiation counter, which is particularly advantageous because of its simplicity and reliability, in addition to ease of disassembly for cleaning and decontamination. Various forms of flow counters for such purposes are known, one exemplary construction being shown in a paper in Analytical Chemistry, volume 36, page 1695 (July 1964), among a number of others.

A serious limitation on such counters as heretofore known is the necessity or desirability, in many instances of gas chromatography use, of operating at relatively high temperatures, both as regards the ow-gas itself, and as regards the ambient environment, which frequently must involve performing the measurement in an oven or similar location. Generally speaking, radiation detectors for 0peration at high temperatures have long been known and used for many purposes, but these have in general employed insulating seals of a -complexity relatively impractical for a chromatograph flow-counter detector. Prior to the present invention, the use of the most desirable flowcounter detectors has been largely restricted to temperatures substantially below 200 degrees centigrade.

It was the original object of the present invention to provide a satisfactory solution to the problem of temperature limitation in simple constructions of flow counters, particularly flow proportional counters for gas chromatography. However, the study of the reasons for such temperature limitations, and the provision of a satisfactory solution for this type of use, have at the same time provided a new type of structure which also may advantageously be used in other types of detectors where operation over an extended temperature range is an important speciiication.

As has long been known, the most readily observed limitation on temperature of operation of a proportional counter or similar radiation detector lies in the hightemperature characteristics of the insulator structure employed. One obvious limit, of course, is the melting point of the insulator. At one time, the commonly available suitable insulating materials Were of relatively low melting point, suchas polystyrene or polyethylene. However, the insulating material now most commonly used, tetrafluoroethylene (frequently designated by the trademark Teflon), has a melting point sufliciently high to be of little concern for many practical purposes. But it is found that constructions of a simple nature using such an insulator are nevertheless unsuitable for use at temperatures substantially below the melting point. A counter using previously known simple constructions is found to develop substantial leaks upon operation at relatively moderate high temperatures, particularly with the cycling between room temperature and elevated temperature involved in non-continuous operation of the gas chromatography equipment in which it is incorporated. The reason for this has been found to lie in a number of closely related effects. First of all, although the melting point is relatively high, there will occur, particularly with the application of large mechanical forces, a certain amount of cold flow, particularly at elevated temperatures. (It will of course be understood that the melting point herein referred to is the approximate temperature at which the insulating material obviously departs from the character of a solid,

`of expansion which increases substantially with temperature and which is, particularly at elevated temperatures, substantially higher than that of the metals employed in the conducting portion of a counter. Accordingly, when a tight seal is made at room temperature, the raising of the temperature to high values produces cold flow distortion of the insulator, which is of course much less resistant to such ow than the metal portions. Thus the restoration of room temperature does not restore the original conditions, but leaves a condition wherein the gas seal is broken. Repeated cycling, of course, tends to exaggerate this relative distortion of the metal and insulating parts.

The present invention, in essence, extends the permissible operating temperature range by providing a structure in which this cold flow or distortion is minimized by permitting the differential expansion to occur in a manner minimizing the resultant stressing or squeezing of the insulator, and thus minimizing its deformation by extrusion-like forces, and also, conversely, when cooling occurs, with resultant relative shrinking of the insulator, restoring the original relative condition and automatically compensating for any small residual distortion of the insulator which may have occurred by providing a new relative location of the parts.

Both the broad principle of the invention, and its narrower aspects, may best be understood from consideration of the embodiment of the invention illustrated in the annexed drawing, and described below.

In the drawing:

FIGURE 1 is a longitudinal sectional view (with the central portion shown in elevation) of a gas chromatography flow proportional counter made in accordance with the invention;

FIGURE 2, is a fragmentary enlarged detailed view of a joint between a metallic cathode and an insulator plug or cap employed in the construction, showing in dotted form, more or less schematically, certain changes in relative positioning occurring in response to temperature changes; and

FIGURE 3 is an enlarged detail view corresponding t0 a portion of FIGURE l, showing the anode connector terminal portion of the construction.

The illustrated proportional counter employs a circular cylindrical tubular cathode with gas flow tubes 12 and 14 entering radially. The ends of the cathode 10 are formed with internal frusto-conical seats 16 and 18. The gas enclosure and counting volume is sealed by insulator caps or plugs 20 and 22, each having a bevelled or conical forward edge 24 and 26 seated on the respective seat 16 or 18.

The insulator cap or plug 20 has an outwardly extending axial extension 28 of reduced diameter, forming a shoulder 30 with the inner portion which bears the conical seat or edge 24.

An end-cap or extension nut 32, of brass or other suitable material, is threaded onto the outer surface of the end of the cathode 10 at 34, with a central aperture 36 freely passing the axial extension 28 of the insulator, and with an inward flange 38- surrounding this aperture. A helical spring 40 is compressed between the ilange 38 and a washer 42 which is on the shoulder 30.

It will be seen that with this structure, the spring 40 urges the conical edge or sealing surface on the insulator 20 into constant engagement with the mating seat on the end of the cathode 10. The changing relation of the parts at this seat with changes in temperature is shown (in somewhat exaggerated form for clarity) in FIGURE 2.

as the're shown by the dotted indications, the excess of expansion of the insulator as compared with the metallic portion upon elevation of the temperature is absorbed by outward motion of the insulator, further compressing the spring. In the absence of any deformation, the original condition is re-established upon return to room temperature. Should there be any small residual deformation, the new position, upon return to the original temperature, will be slightly inward of the original position, but the seal will be unimpaired.

The cap or plug 22 at the opposite end has a somewhat longer axial extension 28a, but the construction of the metal end cap or extension 32a, threaded at 34a, and apertured at 36a, with a flange at 38a, and the helical spring at 40a acting between the flange and a washer 42a, are the same as above described for the correspondingly numbered elements (without the suffix a) already described. The main enclosure is accordingly provided with seals at both ends.

The insulator cap or plug 20 has a well or bore 44 which is threaded at the upper end to receive a similarly threaded bushing or nipple 46, again preferably of Teon. A small conical spring 48 is seated on the outer end of this bushing, and mounts the end of an anode wire 50 which extends back through the conical spring and the bushing 46 and along the axis of the counter body in the usual manner of a proportional counter.

The cap or plug 22 has an axial well or bore 52 on the inner portion thereof, joined by an aperture 54 of reduced diameter which extends longitudinally through the rest of the length of the insulator plug, forming an internal inwardly facing shoulder 55 at the bott-om of the bore 52. A tube 56, of brass or similar material, is slideably mounted in the aperture 54 and is aliixed at its upper end to a terminal cap assembly 58. The terminal cap assembly has a plug or body portion `60v which has a conically concave inner end 62, mating with a conical edge `64 on the reduced-diameter end portion 66 of the insulator plug or cap 22. A threaded joint at 68 makes the upper end of the tube 56 essentially integral with the body 60 of the terminal assembly. A collar 70 at the lower end of the tube 56, within the well or bore 52, compresses a spring 72 against a washer 74 on the shoulder 55, thus springbiasing the interface of the conical surfaces 62 and 64 at the top, to effect a temperature-independent gas seal in this region.

The plug or body 60l of the terminal cap asembly has an axial aperture at 76, and a clamping plate 78, secured by a screw 80, holds the end of the anode wire, at the same time making electrical connection thereto. A connector cap 82 is joined to the top of the plug or body 601 by a pipe-joint connection, thus completing both the gas-tight seal and the external electrical connection of the anode.

In the illustrated embodiment, the conical interfaces at which the temperature-independent gas seals are made employ a bevel angle of 45 degrees. The exact angle is of course not highly critical, and indeed, it will be seen that the advantages of the present structure may be partially obtained without the use of the conical or bevelled mating surfaces. However, best results are obtained with such a conical or bevelled construction, particularly with a properly selected angle of bevel with respect to the axial or seating direction. A bevel which is too small may fail to produce the desired sliding motion in response to differential expansion, while one which is too large unduly reduces the length of the leakage path and the security of the seal. A bevel or cone angle of from 30 degrees to 60 degrees is desirable. Each spring is of course designed to exert adequate force over a range of change of its length sufficient for relatively long lifetime of the sealing assembly, the parts of which may of course be easily replaced in any event.

With the construction illustrated and described, fully reliable operation is readily obtained up to 300 degrees centigrade or higher.

It will be observed that temporary or permanent displacement of the anode terminal with respect to the opposite end of the anode (corresponding to expansion of the springs 40, 40a, and 72) tends to change the tension on the anode wire. The conical anode support spring 48 is of course of less force than the other springs and is shaped to maintain this tension fairly constant over a substantial range of motion. If desired, the tension may nevertheless be readjusted by merely loosening the screw 80.

The embodiment of the invention illustrated in the drawing and described above will readily suggest to persons skilled in the art a large number of variations and modifications, substantially diiferent in appearance and detail, but nevertheless utilizing the basic teachings of the invention. The scope of the protection to be aorded the invention should therefore not be limited by the particular embodiment shown and described but should be determined in terms of the definitions of the invention set forth in the appended claims, and equivalents thereof.

What is claimed is:

1. In a radiation detector of the type having a gas enclosure having a conductive portion, an insulating portion, and an electrode-connection conductor extending out through the insulating portion, the improved construction for use over a wide temperature range characterized by an aperture in said conductive portion formed with a bevelled insulator seat, an insulator having a temperature coeflicient of expansion greater than that of the seat and having a mating bevelled portion seated therein, and a resilient spring acting between the conductive portion of the enclosure and the insulator .and holding the insulator in said mated relation to maintain a gas seal for the enclosure despite repeated cycling of the temperature.

2. The radiation detector of claim 1 wherein the material of the insulator is tetrafiuoroethylene.

3. The radiation detector of claim 1 wherein the bevel forms an angle of from 30 degrees to 60 degrees with the direction of seating.

4. In a radiation detector comprising a iirst conductive electrode element forming an enclosure having an opening therein, an insulator seated in the opening, and a second conductive element having an electrode portion within the enclosure, a connecting portion extending through the insulator, and an external terminal portion, the insulator being in gas-sealing engagement with both the electrode elements, the improved construction for use at elevated temperatures having at least one spring acting between the insulator and one of the electrode elements and urging them into sealing engagement, the spring producing substantial force over a substantial range of change of length, so that the sea-l is maintained despite exposure to temperatures producing relative dimensional changes of the insulator and the conductive element forming the enclosure.

5. The radiation detector of claim 4 wherein the sealing engagement comprises mated frusto-conical surfaces on the insulator and the electrode element, so that the integrity of the seal is not injured by differential expansion.

6. The radiation detector of claim 5 wherein the first electrode element is tubular and has a conical seat formed in the end thereof, the insulator having a corresponding conical portion fitting therein.

7. The radiation detector of claim 6 having an extension member on said tubular end, said extension member having a spring-engaging portion longitudinally spaced from the conical seat, the spring being a helical spring compressed between said spring-engaging portion and an outwardly facing surface of the insulator.

8. The radiation detector of claim 5 wherein the terminal portion and the outer end portion of the insulator have said mated frusto-conical surfaces, the terminal portion having an axial extension into the insulator, and a spring within the insulator urging the axial extension inward to urge said surfaces into engagement to form a gas seal between the insulator and the terminal.

9. A high-temperature ow counter comprising a circular tubular cathode having frusto-conical seats formed in the ends, thermoplastic insulator caps having frustoconical inner end portions matingly seated on the seats and outwardly extending axial stem portions, a generally tubular extension member secured to each end of the cathode having an inwardly facing shoulder in the outer region thereof, a helical spring in each extension member surrounding the stem portion and compressed between the shoulder and the inner portion of the insulator, an axial aperture in one of said insulator caps, a terminal-cap conductor having the periphery thereof in frusto-conical mating engagement with the end of the stem portion a-nd having an axial portion extending into sa-id aperture, an inwardly facing shoulder formed in said aperture and an outwardly facing shoulder on the inner end of said terminal-cap axial portion, a coiled spring surrounding the inner portion of said axial portion and compressed between said shoulders, a spring of much smaller force than said other springs mounted axially on the other of said insulator caps within the enclosure formed by the cathode and the insulator caps, an axial anode wire having one end mounted by said last spring and the opposite end secured to the terminal-cap, and means for owing a gas through the interior of the cathode.

References Cited UNITED STATES PATENTS 2,009,132 7/1935 Gehris 220--55 3,015,729 1/1962 Spaa etal Z50-83.6 3,297,897 l/196 7 Lewis et al 313--247 X I AMES W. LAWRENCE, Primary Examiner.

PALMER C. DEMEO, Assistant Examiner.

U.S. C1. RX. 

